How to Use a Magnetic Stirrer Temperature Probe

Temperature control can be crucial in the research lab when establishing procedures aimed at developing new products and processes.  Mixing and stirring are commonly applied during these activities.  Benchtop hotplate magnetic stirrers allow researchers to exercise close control over sample temperature during their experiments.  A magnetic stirrer temperature probe dramatically increases temperature measurement accuracy and provides additional benefits.

In the following paragraphs we describe:

  • How magnetic stirrers operate
  • Hotplate magnetic stirrers
  • Benefits of a magnetic stirrer temperature probe

 A Brief Introduction to Magnetic Stirrers

A magnetic stirrer is startling in its simplicity. The only moving part is a rotating magnet in the base of the unit, the speed of which can be controlled by the operator.  The rotating magnet is located below a stationary plate on which sample beakers and flasks are placed.

The rotating magnetic in the unit causes a coated bar magnet in the sample container to rotate and perform the mixing action  Stirring speeds depend on the model but can reach 1600 RMP in equipment offered by CAT Scientific.

This differs from laboratory homogenizers and overhead mixers where mixing is accomplished by motor driven generator assemblies and shaft impellers. The limitation is that there is no physical connection between the beaker magnet and the rotating magnet in the stirrer’s base – restricting use to relatively low viscosity samples.

Hotplate Magnetic Stirrers

A hotplate magnetic stirrer allows researchers, as you might imagine, to heat samples during the stirring cycles.  Temperatures can be controlled to as high as 450⁰C (842⁰F) in CAT Scientific hotplate magnetic stirrers.

When programming processing speed and processing time researchers also program processing temperature in hotplate magnetic stirrers.  The point is, this is the hotplate temperature, not the sample temperature.

The difference between plate temperature and sample temperature may be important.

Also important is sample evaporation or loss due to beaker failure when samples are at such high temperatures.

Benefits of a Magnetic Stirrer Temperature Probe

Hotplate magnetic stirrers such as the CAT MCS78 provide two means of temperature control.  Optimal performance in terms of temperature control and overall operation is provided when an external temperature probe is used.

Designated as the Pt100 probe in our literature, these are described as a platinum resistance thermometer (PRT) or a resistance temperature detector (RTD). Because of their accuracy and repeatability they are replacing thermocouples as a means of gauging temperature.

The principle of operation is to measure the resistance of a platinum element over the operating temperature range of the magnetic stirrer. Wide temperature ranges such as the 440⁰C upper limit of the CAT MCS78 model magnetic mixer can easily be accommodated with the Pt100 probe.

While temperature probes can be simply inserted in the mixing vessel a better option is affixing them to a clamp or support rod so the tip is not in contact with the bottom of the sample flask.  This assures that the probe will not be nudged by the spinning stirring magnet or get an incorrect reading because of its proximity to the hotplate.

How to Operate Magnetic Stirrers with Pt100 Temperature Probes

Full operational instructions are provided with each CAT Scientific magnetic stirrer.  But in brief, here’s how to set things up using our MCS78 model:

  • Connect the external Pt100 probe to the socket marked Pt100 at the rear of the unit.
  • Put the Pt100 probe into the vessel on the hotplate and immerse it at least 2cm in the liquid
  • Press power on/off to switch the unit on
  • Press the probe temperature button to set the desired probe temperature (ProbeSet) with the up/down keys
  • Press the plate temperature button to set the maximum allowed hotplate temperature (PlateLim) for the heating process. Note: This temperature should always be set as high as possible to allow the control algorithms to freely select the optimum hotplate temperature. The PlateLim temperature must always be set at least 10%-20% above the programmed probe-temperature.  Otherwise the liquid cannot reach the programmed temperature or the heat up time would become extremely long.
  • Press plate on/off to activate the hotplate (green control LED must be on)
  • Press motor on/off to switch on the stirring motor

The MCS78 magnetic mixer will automatically select the fastest optimum hotplate temperature to reach the desired liquid temperature.

Temperature Control Without a Probe

  • Press power on/off to turn the unit on
  • Press plate temperature button and select the hotplate temperature using the up/down keys
  • Press plate on/off to activate/deactivate the hotplate
  • Press motor on/off to activate/deactivate the stirring motor

Safety Features

Another benefit of using temperature probes is their contribution to the safe operation of CAT magnetic stirrers by initiating shutdown procedures in the event that

  • The Pt100 tip is not immersed in liquid
  • If there is a discrepancy between the probe temperature and the hotplate
  • If there is no increase in liquid temperature over a certain time but the hotplate temperature is rising
  • If the probe is accidentally disconnected from the back of the unit

There are other safety features built into CAT magnetic stirrers.  For information visit our post on CAT Magnetic Stirrer Safety Features.  Or if you have questions, please send them to us with our Ask A Question form.

Bob Wilcox

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid handling and related laboratory equipment since 2002 when Staufen, Germany-based CAT Ingenieurbüro M. Zipperer GMbH established operations in North America. Bob oversees CAT Scientific laboratory apparatus sales and service organization from the company’s headquarters in Paso Robles, CA. He also is in charge of the parent company’s line of JetCat jet turbines, turboprop, and helicopter power plants for hobbyists’ radio controlled fixed wing and helicopter model aircraft. -- Earlier in Bob’s career he was involved in visual and special effects as well as camera and electronics supervisory responsibilities for the motion picture and television industry.

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